Optical sensing systems may use optical amplifiers to compensate for signal attenuation. Two types of optical amplifiers are a phase insensitive amplifier (PIA) and a phase sensitive amplifier (PSA).
An example PIA is an erbium-doped fiber amplifier. An advantage of a PIA is that signal gain is produced independent of the signal phase. This means that the phase of the PIA pump does not need to be locked to a local oscillator signal. However, a PIA adds noise to the optical signal being amplified.
In contrast, with coherent detection, a PSA may advantageously amplify the optical signal without adding noise. However, to realize noiseless amplification, the phase of the PSA pump needs to be locked to a local oscillator signal.
One approach for using a PSA is disclosed in U.S. Published Patent Application No. 2007/0216994, wherein a PSA is produced by degenerate four-wave mixing (FWM) in a randomly-birefringent fiber (RBF). In particular, the PSA includes a non-linear optical fiber, a first pump to input polarized electromagnetic energy having a first polarization angle and a first wavelength into the optical fiber, and a second pump to input polarized electromagnetic energy having a second polarization angle and a second wavelength into the optical fiber. An optical communication signal source inputs into the optical fiber a polarized optical communication signal having a third polarization angle between the first and second polarization angles and a third wavelength between the first and second wavelengths. Vector FWM near the zero-dispersion frequency (ZDF) of the RBF produces phase-sensitive amplification, which makes it easier to phase-lock pumps with similar frequencies, as produced by a single laser and phase modulator.
In other embodiments of an optical sensing system interfacing with an optical fiber, phase locking of a PSA has been performed via injection locking techniques. When the phase and amplitude of the optical signal being amplified are known, various phase locking and carrier extraction schemes may be used to support a PSA. These schemes allow the phase of the optical pump source associated with the PSA to be phase-locked to a local oscillator signal.
For free-space optical signals, PSAs have also been used in imaging and light detection and ranging (LIDAR) applications using direct detection, but the PSA is operated in such a way that the amplification is not noiseless. Knowledge of the phase is not necessary in direct detection. However, there is a need to be able to operate a PSA with coherent detection so that amplification may be substantially noiseless.